High-temperature stainless steel



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Application any is, 1m, Serial No. 21,15:

1 Claims. (01. is-1:5)

The present application is a continuation-inpart'ot my copending application, Serial No. 671.905. filed May 23. 1946, now Patent No. 2,447,897. and entitled High Temperature Stainless Steel, which application in turn is a continuation-in-partof my copending application, Serial No. 845,015, filed February 1, 1946. now Patent No. 2,447,896, and entitled High Temperature Turbins, and the invention relates to high temperature steel and products and articles of the steel.

An object of my invention is the provision of strong anddurable high temperature stainless steel, and products and articles of the steel. which areresistant to stress-rupture and creep while under load in the heated condition. and which resist attack of hot corrosive matter and avoid the formation of heat scale up to very high temperatures. a A further object of this invention is the provision of high temperature stainless steel having excellent hot working properties, great physical strength and high resistance to corrosion; another obiect being that'of providing wrought products and articles of the steel.

A still further object is the provision of high temperature stainless steel subjected to heat treatment for erties.

Other objects in part will be obvious and in part pointed outmore fully hereinafter.

The invention accordingly consists in the combination of elements, composition of materials. and in the articles. products and manufactures. and in the several method steps and the relation of each of the same to one or more of the others as described herein, the scope of the application of which is indicated in the following claims.

' A's conducive to a clearer understanding of certain features oi my invention, it may be noted at this point that a great variety of austenitic stainless steels in the prior art, as for example those containing about l% to chromium. 7% to 15 or more nickel, and the remainder substantially all iron, are usefullin the production of low temperature or mildly heat resistant products orarticles-of. manufacture. Among the better known ofthesesteelsare the usual 18-8 chromium-nickelsteels which are readily worked as by rolling, drawing.,spinning, or the like, into such products as t'rimfor house, store, omce or restaurant purposes or into vehicle or outdoor trim subjected to the effects of weather. Then too, these steels are very much in demand for the fabrication into products, such as for cooking and table service. receptacles and other appliances enhanced high temperature prop- 2 where a metal of enduring brightness is called for even to withstand body salts. fruit acids and other corrosive chemicals.

The austenitic stainless steels of the character just referred to have a relatively high alloy content by virtue of the inclusion of nickel, as compared with straight chromium ferritic stainless steels, and are relatively better suited to resist oxidation when heated to high temperatures, say for example above 1000' I". The nickel, importantly being present as an austenite former, contributes materially to the oxidation resistance. There is still the existing fact. however, that the austenitic stainless steels ordinarily on the market are unsatisfactory for meeting exacting demands at high temperature as in such forms as bolts and fasteners, internal combustion engine valves, gas and steam turbine blades, rotors, buckets, nozzles, supercharger components, or in go the form of a host of other products and articles subjected to mechanical stress under temperature conditions of use. Products when made of these steels are too susceptible to creep and stress rupture while hot.

as It remains that austenitic stainless steels, as a general class, have a more favorable lattice structure for cohesion under stress or load at high temperatures than do ferritic straight-chromium stainless steels. This alone, however, does not necessarily assure adequate high temperature properties in austenitic steels as an entire class. especially with respect to the considerations of stress rupture and creep. The austenitic steels in most instances do have favorable hot working properties, which so often is not true of cerstress-rupture under mechanical load and severe heat.

Referring now more particularly to the practice of my invention, I find that by correlating proper amounts of the ingredients chromium.

nickel, cobalt, manganese, molybdenum, copper, carbon, and either or both titanium and columbium, to provide an austenitic stainless steel, satisfactory hot working properties are had along with excellent high temperature strength and re-- sistance to heat and corrosion. The steel which I provide preferably contains about 12% to 22% chromium, to 22% nickel, cobalt ranging from 8% to from very smallamounts up to 2% manganese, 2% to 4% molybdenum. 2% to 4% copper, from 0.08% to 0.20% carbon. at least one metal of the group consisting of titanium and columbium. the titanium ranging from 0.15% to 0.75%. the columbium from 0.20% to 1.10%, and the remainder substantially all iron. Such elements as sulphur and phosphorus preferably are less than about 0.04% each.

I often form my steel, as by not working and machining, into any of a host of high temperature products and articles. among which are bolts and fasteners, rivets, high temperature chemical equipment parts, tubes such as seamless tubes formed by piercing and drawing. gas and steam turbine blades, rotors, buckets, nozzles. and supercharger parts for either intermittently or continuously serving under mechanical stress and/or corrosive attack. The high temperature products and articles of manufacture are strong. durable and reliable. They resist creep and stress-rupture while hot.

In certain instances, I provide products of the steel in such form as sheet. strip. wire. rods. or the like. which are readily useful for fabrication as by cutting. punching or bending into shape. or welding as by means of oxy-acetylene or arc welding equipment. Sometimes, too, I use the sheet, wire, or the like, directly in high temperature applications without appreciable fabrication.

My high temperature stainless steel and articles and products thereof are wholly austenitic. Ferrite, if present at all, is only in traces. This I find is essential to the required stress-rupture properties. Where appreciable amounts of ferrite are present the stress-rupture values fall 01!; also. the working properties suffer.

The substantial quantity of cobalt present in my stainless steel not only supplements the effect of nickel to assure an austenitic balance, but improves the stress-rupture properties and seems to arrest development of sigma phase. Also, by reason of the cobalt constituent, a steel of relatively high alloy content is had in favor of better scaling resistance under intense heat. With, any appreciable lowering of the molybdenum and copper contents to outside the ranges hereinbefore noted, the desired high temperature loadcarrying capacity of the steels suiIer, and with appreciable increase beyond the ranges workability disappears. The elements titanium' or columbium, or preferably both, likewise constitute a desired part of the alloy. Alone or together as the case may be, and especially with proper heat treatment of the steel, they enhance stress-rupture and creep properties and give improvements in the high temperature loadcarrying ability.

The particular form of heat treatment which I employ for enhancing the high temperature loadcarrying properties of the steel is in the character of an annealin and precipitation treatment. In the preferred embodiment of this practice, I heat up the steel, as for example roughly a formed articles of the steel, to within solution temperature range of about 2050 F. to 2250 I". at which temperatures either or both the titanium and columbium go into solution depending upon whether one or both are present. The titanium is quite soluble throughout the heating range. The solubility of columbium increasestowardtheupperendofthetemperature range. Copper also goes into solution. working operations are achieved on the metal if desired. either before, along with. or following the annealing treatment.

After a suitable period of time, I terminate the treatment at solution temperature and quench the steel as in air. oil or water conveniently to about room temperature. By subsequently heating the quenched metal preferably to within a temperature range of about 1200 F. to 1500 F. and. by maintaining the same at temperature for a long enough time. a critically dispersed finely divided precipitate comes out in the metal latticealongtheslipplanesinthematrix. Inthis, there is a precipitation of intermetallic compounds including titanium or columbium or both. These come out in fine form, or possibly as an intermetallic compound including such elements as nickel. molybdenum. titanium and/or columbium. Although not immediately visible with the ordinary light microscope, they become visible after considerable time is allowed for coalescing. some portion of the elements precipitated, such as the titanium and columbium. is in the form of carbides which increase in amount on the higher side of the precipitation treatment temperature range lust noted.

I quench the steel from the precipitation heattreated condition. The quenched metal has a fine grain structure. and is further characterized by enhanced load-carrying capacity in view of atomic slip interference developed by the precipitates. The steel in this condition often is worked. fabricated or finished to give articles or products. During hilh temperature use of the steel, the precipitates remain critically dispersed. uncoalesced and eifective against creep and stress-rupture, for long periods of time. Any heating of the metal to as high as the solution temperature, of course, tends to put the precipitates back into solution.

A few examples of the austenitic chromiumnickel-cobalt stainless steels which I provide are identified in Table I. These steels in addition to containing the amounts of ingredients noted have a remainder which is substantially all iron.

Table L-Chromium-nickel-cobalt Several samples of the steel x were prepared, then were given annealing treatment at somewhat dlil'erent temperatures followed by identical precipitation heat treatment, and then were tested with respect to stress-rupture and the cor-- responding elongation and reduction in area. Table II gives the results of these tests for the several conditions of heat-treatment.

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Table "Aimsun tests of I steel 1: at 1200 1'. [Condition A annealed at 2050 F. lo! )4 hour and water-quenched plus aging at 1200' 1'. lot 5 hours and water-quenched] Load Time mim i Reduction Rupture in Area P. I. i. Hours Per cent 43.1111- 81. 6 i ii. 3 l3. 5

40. (X!) 238. 3 l2. 2 l0. 8

[Condition plus Upon subjecting samples of the steel Y to stress-rupture tests, the results noted in Table III were had.

Table llL- stress-rupture tests of steel Y [Steel annealed at 2250 F. for 36 hour and water-quenched plus aging at 1350" F. for hours and water-quenched, before testing] Elonga- Time for Reduction Rupture 92. in Area TESTED AT 1210' I".

P. s. i. Hmm Per cent TESTED AT 1500 F.

15, (I!) 172 8. 14 ll. l8

12, (I10 583 I 2. 72 3'. so

Tats also were made on samples or the steel z, these samples being annealed and precipitation heat-treated according to the schedule in Table IV.

Table IV.Stress-rupture tests of steel 2 [Steel annealed at 2250" r. (or 5 hour and "melanoma plus aging at 1350 F. for 5 hours and water-quenched, beiore testingJ Elonga- Time for Reduction Md Rupture 99% in Area TESTED AT 1200 F.

P. I. i Hours Per cent 50, 000 145. 7 l3. 0 20. 0 48, (I10 Zil. 0 ll. 2 l5. 5 46, Oil) 454 8. 3 19. 3

TESTED AT 1500" F.

My austenitic chromium-nickel-cobalt stainless steels have many valuable high temperature properties, including resistance to stress-rupture and resistance to creep whether or not the steelsandproductsandarticlesthereoiarein the precipitation heat-treated condition. The precipitation heat treatment enhances a number of the properties by the development of atomic slip-interference. My austenitic steels despite the high temperature properties of the same are capable of fabrication in a simple, direct and economical manner by virtue of the ease of working.

While the carbon content of my steel preferably is on the order of 0.08% to 0.20%, many desirable properties nevertheless are had where the carbon content is as low as 0.03%, or even lower.

Thus it will be seen that there are provided in my invention austenitic chromium-nickel-cobalt alloy stainless steel and products thereof in which the various objects notedtogether with many thoroughly satisfactory results are successfully achieved. It will be seen that the products are tough, strong and durable. corrosion-resistant and heat-resistant and serve well at high temperatures over long periods of time under many conditions of actual practical use.

As many possible embodiments may be made of my invention and as many changes may be made in the embodiment hereinbefore set forth, it will be understood that all matter described herein is to be interpreted as illustrative and not as a limitation.

I claim:

1. Austenitic chromium-nickel-cobalt stainless steel having good hot-working properties and good. life and strength at high temperatures, containing about 0.03% to 0.20% carbon, 12% to 22% chromium, 10% to 22% nickel, from 5% to 20% cobalt, not more than about 2% manganese, 2% to 4% molybdenum, 2% to 4% copper, at least one metal of the group consisting of titanium and columbium, the titanium ranging from 0.15% to 0.75% in amount. and the columbium from 0.20% to 1.10%, and the remainder substantially all iron.

2. Austenitic high temperature precipitationhardened stainless steel having great strength and great resistance to creep at high temperatures, containing about 0.08% to 0.20% carbon, 12% to 22% chromium. 10% to 22% nickel, from 5% to 20% cobalt, 2% to 4% molybdenum, 2% to 4% copper, not more than about 2% manganese. 0.15% to 0.75% titanium, 0.20% to 1.10% columbium. and the remainder substantially all iron, said nickel, molybdenum, titanium and columbium being at least partially precipitated in critically dispersed form for slip-interference.

3. Austenltic high temperature precipitationhardened stainless steel having great strength and great resistance to creep at high temperatures, containing about 12% to 22% chromium. 10% to 22% nickel, from 5% to 20% cobalt, 2% to 4% molybdenum, 2% to 4% copper, 0.03% to 0.20% carbon, not more than about 2% manganese, from 0.15% to 0.75% titanium, and the remainder substantially all iron, said titanium being at least partially precipitated in critically dispersed form for slip-interference.

4. Austenitic high temperature precipitationhardened stainless steel having great strength and great resistance to creep at high temperatures, containing about 12% to 22% chromium. 10% to 22% nickel, from 5% to 20% cobalt, 2% to 4% molybdenum, 2% to 4% copper, 0.03% to 0.20% carbon, not more than about 2% manganese, from 0.20% to 1.10% columbium, and the remainder substantially all iron, said columbium.

being at least partially precipitated in critically dispersed form for slip-interference.

5. Austenitic chromium-nickel-cobalt stainless good life and strength at high temperatures, containingabout 14% to 16% chromium. 10% to 12% steeihavingg'oodhotworkingpropertiesand good life and strength at high temperatures.

containing about 10% to 10% chromium, 12% a.

14% nickel. 12% to 14% cobalt. 2.5% to 3.5% molybdenum, 2.5% to 8.5% copper, at least one oi the group consisting of .15% to 35% titanium and 20% to 1.10% oolumbium. 0.08% to 0.20% carbon, and the remainder substantially all iron.

6. Austenitic chromium-nickel-cobalt stainless steel having good hot working properties and good life and strength at high temperatures, containing about 20% to 22% chromium. 13% to 15% nickel, 15% to 17% cobalt. 2.5% to 8.5% molybdenum, 2.5% to 3.5% copper. at least one of the group consisting of .15% to .7595 titanium and 20% to 1.10% columbium. 0.08% to 0.20% car'- bon, and the remainder substantially all iron.

I. Austenitic chromium-nickel-cohalt stainless steel having good hot working properties and nickel. 15% to 17% cobalt, 2.5% to 3.6% molybdk nurn. 2.5% to 3.5% copper, at least-one oi the group consisting of .15% to 315% titanium and 20% to 1.10% columbium. 0.08% to 0.20% carbon. and the remainder substantially all iron.

WIILIAM CHARLEQ CLARKE, Jr.

REFERENCES CITED The following references are oi record in the tile 01' this patent:

UNITED s'm'rns PATEN'IB Number Name Date 2,307,034 Mohling Mar. 10, 1046 2,447,896 Clarke Aug. 4. 1948 2,447,897 Clarke Aug. 24. 1048 

1. AUSTENITIC CHROMIUM-NICKEL-COBALT STAINLESS STEEL HAVING GOOD HOT-WORKING PROPERTIES AND GOOD LIFE AND STRENGTH AT HIGH TEMPERATURES, CONTAINING ABOUT 0.03% TO 0.20% CARBON, 12% TO 22% CHROMIUM, 10% TO 22% NICKEL, FROM 5% TO 20% COBALT, NOT MORE THAN ABOUT 2% MANGANESE, 2% TO 4% MOLYBDENUM, 2% TO 4% COPPER, AT LEAST ONE METAL OF THE GROUP CONSISTING OF TITANIUM AND COLUMBIUM, THE TITANIUM RANGING FROM 0.15% TO 0.75% IN AMOUNT, AND THE COLUMBIUM FROM 0.20% TO 1.10%, AND THE REMAINDER SUBSTANTIALLY ALL IRON. 